Follow-on to #2069 - Decrease stack size of MPTT rebuild task.
This reduces the likelihood that we'll hit a limit with gargantuan galleries.

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extra : source : c4a118d43145a2a4ec9b934d3aebe8f9458bcf07
Merge pull request #371 from shadlaws/followon2069_master

Follow-on to #2069 - Decrease stack size of MPTT rebuild task.